Energy efficiency and reducing the fuel consumption of motor vehicle engines is a growing concern. It is known that lubricants for engines used in said vehicles play an important role in this regard.
In order to formulate “Fuel Eco” lubricants or fuel economy lubricants, it is known to act on the viscosity of the lubricant bases used. It is also known to use viscosity index (VI) improver polymers, or to use friction modifiers (FM). However, the viscosity index improver polymers have the drawback of reducing the engine cleaning power of the lubricant compositions in which they are used. Existing engines have high thermal stresses which result in significant deposit phenomena. The deposits are linked to the chemical conversion of the lubricant in the parts that are closest to the combustion chamber and therefore the hottest.
A need therefore exists to have lubricant compositions comprising at least one viscosity index improver polymer which provide good engine cleanliness and which make it possible to limit the fuel consumption of gasoline vehicles or diesel vehicles. An objective of the present invention is the use of new additive compounds in a lubricant composition making it possible to formulate a lubricant composition having good properties in terms of engine cleanliness. This objective is achieved by means of the use of at least one polyalkylene glycol, obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, including at least one butylene oxide in a lubricant composition. Surprisingly, the Applicant company has found that the use of these polyalkylene glycols as additives advantageously makes it possible to obtain a lubricant composition having good properties as regards engine cleanliness.
Another objective of the present invention is the formulation of a lubricant composition simultaneously having good engine cleanliness properties and good “Fuel Eco” properties. This objective is achieved by means of a lubricant composition for engines, in particular gasoline engines or diesel engines, comprising a specific combination of a polyalkylene glycol obtained by polymerization or copolymerization of alkylene oxides, including at least one butylene oxide and at least one viscosity index improver polymer.
Such propylene oxide and butylene oxide copolymers are known from the document WO2011/011656. These propylene oxide and butylene oxide copolymers have the property of being soluble in the base oils of Groups I to IV used in the formulation of the lubricants.
The document U.S. Pat. No. 6,458,750 describes an engine oil composition with reduced deposit-formation tendency, said composition comprising at least one base oil and at least one alkyl alkoxylate of formula (I):R1(CR2R3)nzL-A-R4  (I)whereinR1, R2, R3 represents independently one hydrogen atom or a hydrocarbon group containing up to 40 carbon atoms,R4 is a hydrogen atom or a methyl group or an ethyl group,L is a linker group,n is an integer ranging from 4 to 40,A is an alkoxy group with 2 to 25 repeating units, which are derived from ethylene oxide, propylene oxide and/or butylene oxide and comprising homopolymers as well as statistical copolymers of at least two of the said compounds, andz is 1 or 2.However, this document does not disclose a lubricant composition comprising at least one polyalkylene glycol which is a copolymer of butylene oxide and propylene oxide in which the butylene oxide to propylene oxide mass ratio is selected from the range of values of the present invention. In addition, this document does not describe the use of a specific polyalkylene glycol for improving the engine cleanliness, without increasing the consumption of gasoline or diesel fuel.
The document EP0438709 discloses an engine oil comprising at least one base oil, at least one polymeric viscosity index improver and at least one product resulting from the reaction of phenols or bisphenol A with at least one butylene oxide or a butylene/propylene oxide for improving piston cleanliness of automobile engines. However, this document does not disclose the lubricating compositions according to the invention. Nor does it disclose the use of polyalkylene glycol as defined by the invention in a lubricant composition in order to improve engine cleanliness and reduce fuel consumption. In order to simultaneously obtain good “Fuel Eco” and cleaning properties, the quantity of polyalkylene glycol in the lubricant composition must be limited to between 1 and 30% by mass with respect to the total mass of the lubricant composition, the 30% upper limit being excluded.